The invention relates to frequency converters and to electric drives.
An electric motor drive, i.e. electric drive, is an energy converter provided between a supply network and a process for converting, by means of a machine driven by an electric motor, the energy of the network for use by the process. Frequency-controlled cage induction motor drives often employ frequency converters provided with an intermediate circuit. In accordance with FIG. 1, a typical frequency converter provided with an intermediate circuit comprises a rectifier 10 that supplies a pulsating direct voltage to the capacitor battery of a direct voltage intermediate circuit 11 for generating direct voltage in the intermediate circuit. The last component is an inverter 12, whose controllable switch components are used to re-modify the direct voltage of the capacitors of the intermediate circuit 11 into an alternating voltage of the desired frequency. In addition, the frequency converter usually includes a control unit 13 for attending to the appropriate operation of the frequency converter. The amplitude of the output voltage of the frequency converter is typically adjusted by changing the pulse pattern of the output voltage by pulse width modulation, for example.
Many drives always rotate in the same direction and the load never has to be braked. In other words, the power flows from the supply network through the rectifier, the intermediate circuit and the inverter to the motor. However, the power (e.g. braking energy) cannot flow through a conventional rectifier 10 from the motor to the supply network. A four-quadrant drive is an electric drive, wherein the power can flow freely from an alternating current supply network to a load and from the load back to the supply network. At the supply network side, the four-quadrant drive also comprises an inverter supply unit 12 implemented with switch components. The switch elements, or choppers, are gate-controlled power transistors (IGBT); fast, so-called freewheeling diodes being connected between the collector and emitter of the transistors. Other examples of switch components include MOSFET and bipolar transistors. The diodes of the inverter supply unit 12 are usually employed also for rectification when power flows from the supply network towards the load. Since the diodes immediately become conductive when a forward bias voltage is provided across them, the four-quadrant drive cannot be connected to the supply network without auxiliary devices with which the intermediate circuit capacitor battery 11 is first charged to the level required by the mains voltage. For this purpose, separate main and charging contactors and one or more current-limiting charging resistors are usually employed.
FIG. 2 shows an example of a four-quadrant drive comprising a circuit for charging the intermediate circuit capacitor battery. Switch module SM1 corresponds to the rectifier 10, and switch module SM2 corresponds to the inverter 12 in FIG. 1. In both switch modules, switch components SW1 to SW12 are for instance gate-controlled power transistors (IGBT); fast, so-called freewheeling diodes D1 to D12 being connected between their collector and emitter. The capacitor battery of the intermediate circuit comprises capacitors C1 and C2. Contactor K1 is the main contactor, dimensioned according to the nominal phase current, and contactor K2 is a charging contactor dimensioned according to the charging current. Resistor R1 is a charging resistor.
In FIG. 2, a star-connected secondary winding M1 of a transformer T1 presents a supply network, from which the intermediate circuit capacitor battery C1-C2 is charged by first closing the charging contactor K2. The capacitor battery C1-C2 is charged through a diode V1 and the current-limiting resistor R1, until control logics 20 of the contactors observe that the capacitor battery C1-C2 has reached a sufficiently high voltage level. This being so, the main contactor K1 is opened, the capacitor battery C1-C2 being charged to its final voltage through the diodes D1 to D6, connected as a three-phase bridge, of the switch module SM1. The charging contactor K2 can now be opened.
At the circuit diagram level, the method seems simple, but high-power contactors and charging resistors are bulky and outstandingly expensive components. In addition, the power required by the pull-through winding of a large contactor in operating the contactor may be hundreds, even thousands of volt-amperes, and the holding power dozens of watts. This requires an extremely effective power source, which is otherwise not necessarily required.
The rectification operation of the switch module SM1 shown in FIG. 2 is called six-pulse rectification, since the direct voltage of the intermediate circuit is composed of six pulses during a mains voltage cycle. It is evident that when a 12, 18 or 24-pulse rectification is desired, i.e. when the number of supply voltage phases is increased, the number of main contactors K1 has to be doubled, tripled or quadrupled.
The diode bridge/switch rectifier according to FIG. 2 is also generally employed not only when an actual four quadrant is required, but also for decreasing the large mains current distortion generated by six-pulse diode bridge rectification, although the power would not have to be fed back to the mains network. However, IGB transistors and fast diodes are quite expensive, and the power loss properties of a mains bridge implemented with fast diodes are not as good as those of a bridge implemented with slower, so-called mains diodes.
The distortion problem can be evaded by employing 12-pulse rectification using the circuit of FIG. 3, for example. In practice, the current tolerance of components is typically such that a rectifier formed of quite low-current and extremely inexpensive thyristor/diode modules 10A and 10B is adequate for supplying the most high-power switch module 12A and 12B implemented with IGB transistors in a low-voltage frequency converter. Even average-power (>200 kW) frequency converters require a parallel connection of two or more switch modules. In this case, it is preferable to supply a power for each switch module 12A or 12B with the dedicated rectifier 10A and 10B, which can be connected to a 6, 9 or even 12-phase mains supply (in FIG. 3, to the star-connected and delta-connected secondary windings M1 and M2), whereby the current distortion reflected to the primary winding (not shown) of the supplying transformer T1 is significantly reduced. In the assembly, a common intermediate circuit capacitor C is preferably employed. The rectifiers are controlled by thyristor control 30. The circuit of FIG. 3 achieves a significantly lower mains current distortion than a six-pulse diode bridge, but power cannot be transferred to supply the network. This is not even required in fan or pump drives. The motor may comprise either one or two windings.